Transportation equipment, and particularly aircraft, utilize a variety of devices and fastening methods in the assembly of structures. Many attachments that must be provided involve the use of inserts such as bushings, nut plates, or plugs. Such inserts are installed into holes, particularly into holes in walls of metal structures. Generally, bushings are applied to pinned joints requiring easy, reliable assembly. Bushings can also be provided in a suitable material selected to provide a hard wear surface; such wear surfaces can be replaceable. Rivetless nut plates are applied to locations where access by a mechanic to one side of the structure is limited or impractical due to the design of the structure. Plugs and repair bushings are applied to discrepant and misplaced holes which occur during manufacturing, as well as to those which have become non-conforming during use. Each of these items presently utilizes a bushing or bushing-like member that is presently installed using any one of a variety of heretofore available methods.
Bushings. One of the simplest methods previously utilized for joining two members together is the use of a single bolt or pin connection. Such a joint can carry relatively large loads yet is easily and quickly connected. It has been generally standard practice to use bushings in the holes of the single bolt or pin type connections to protect against wear at the pin-to-hole interface. However, high cyclic stress and relative rotational motion work to cause fatigue and fretting damage to such joints. Use of a bushing allows a damaged structure to be repaired by simply installing a new bushing into the fitting. Bushings also provide for an increase in bearing strength of a bolt or pin connection by slightly increasing the bearing diameter.
Even with such benefits, structural assemblies with bushed holes have been historically prone to fatigue damage due to high loading and relative oscillatory motion at the bushing-to-structure interface. Additionally, such interfaces as heretofore often found are subject to damage from corrosive elements found in the environment.
A common method for improving the fatigue life of a bushed hole is to install an interference fit bushing. Bushing interference is defined as the geometric difference between the outside diameter of the bushing and the diameter of the hole in which it is installed. Traditional techniques use a liquid nitrogen bath to shrink oversize bushings into holes. Upon warming, the installed bushing expands in an attempt to return to its original diameter, although it is compressively restrained from complete expansion by the edge wall of the hole in which it has been inserted. This action results in an interference fit with the hole. The shrink-fit technique, as it is generally known, is typically limited to a diametric interference of about 0.002 to 0.003 inches, more or less. Attempts to install bushings at higher interference fit levels have often resulted in scoring and galling of the edge wall of the hole, undesirably resulting in reduced fatigue life.
However, one prior art method developed by The Boeing Company (formerly at McDonnell Aircraft Company, St. Louis, Mo., US) installs bushings to a much greater degree of interference than just mentioned, by using a method of hole cold expansion tooling. Initial clearance fit bushings are expanded into holes using a tapered mandrel. That method provides higher interference fit levels (0.004 to 0.008 inches), and results in some improvement in fatigue life.
Nut Plates. A nut plate is a device that can be generally described as a small plate to which a fastener nut is secured, which in turn is secured to a wall of a structure, to facilitate blind attachment of panels or other objects. A blind attachment is conventionally described as one where access to the backside of the wall is not physically accessible, as is the case with removable access panels. Also, nut plates are generally used on thin walls that are insufficiently thick to permit the use of a threaded wall in an aperture. Riveted type nut plates typically include a pair of small fastener apertures spaced diametrically apart on opposite sides of a nut, and a larger, centrally located aperture coinciding with nut placement.
A primary focus of attention for improving nut plate fatigue life performance has been the elimination of the attaching rivet holes. Such a technique generally has a two-fold benefit; first by improving fatigue life by reducing the stress concentration at the rivet locations, and second by simplifying the installation procedure. Designs for the same have been generally called “rivetless” nut plates. Various rivetless nut plate designs have been produced including (1) those with swaged bushing elements with nut attach features, (2) those with epoxy bonded nut plates, and (3) those with conventional looking nut plates that are swaged and that use an anti-rotation pin.
Regardless of the design, i.e., whether riveted or rivetless, nut plates must be constructed and installed so as to reliably resist both the push-out and the rotational forces which are experienced during frequent bolt insertions and removals. A nut plate that fails to resist either of these forces, especially rotation, makes removal of a panel problematic. The resistance to push-out and to rotational forces is generally proportional to the diameter of the nut. An industry standard that has been commonly accepted for the push-out and removal resistance of a nut plate is National Aerospace Standard NASM25027. As an example, a ¼ inch nut plate must be able to withstand (1) a push-out force of 125 pounds force, and (2) a torque force of 100 inch-pounds force.
An early rivetless nut plate concept with enhanced fatigue life performance was developed by the Deutsch Fastener Corporation; the design of which is illustrated in U.S. Pat. No. 4,732,518, issued to R. Toosky on Mar. 22, 1988, and entitled FATIGUE RESISTANT FLARED FASTENER. The main feature of that nut plate is that it has a bushing-like element with integral exterior splines that serve as anti-rotation lobes. The bushing-like element inserts into a controlled starting hole dimension. The starting hole has a diametric tolerance range of 0.005 inches. The starting hole may be optionally counterbored or countersunk on the side opposite the nut, for improved flushness and for resistance to push-out. A sheet metal cage element holds the nut in a floating arrangement; that element is attached to the bushing element. That prior art nut plate is installed by first placing it into a properly sized starting hole then pressing a swaging tool consisting of locally enlarged mandrel and nosepiece. The mandrel portion of the swaging tool expands the bushing element into the hole wall while the swaging nosepiece forces the panel side portion of the bushing element into the wall—typically the counterbored or countersink area of the starting hole. The finished structure made thereby is resistant push-out and rotation forces.
Another version of a rivetless nut plate with a bushing-like element was developed by Fatigue Technology (“FTI”), and is disclosed in U.S. Pat. No. 5,405,228, issued to L. Reid et al on Apr. 11, 1995 and entitled NUT CAGE AND MOUNT. That nut plate is installed by expanding the inside diameter of the bushing element. That is accomplished by pulling a tapered mandrel pre-fitted with a split sleeve through the inside diameter. This process is similar to split sleeve cold working of a hole. The combination of the mandrel diameter and sleeve thickness results in yield of the wall of the bushing and expands it into the hole. Resistance to push-out and rotation is enhanced by a roughened exterior surface. Rivetless nut plates of that design require starting holes with diametric hole tolerances of about 0.002 inches. However, in order to achieve a reliable resistance to push-out and rotation with the FTI nut plate it is necessary to have a consistent interference fit between the outer diameter of the bushing element and the starting hole. Such interference is defined as the geometrical difference between the installed bushing element outer diameter and the original diameter of the starting hole. The high interference of the bushing element to starting hole coupled with the roughened outer surface of the bushing element provides the necessary resistance to push-out and rotational forces of the bolt. Installed diametric interference between the outer diameter of the bushing element and the starting hole typically ranges from 0.004 inches to 0.006 inches. Of course, the resulting interference is dependent on the expansion of the bushing element during installation. Therefore, the tolerance stackup, and subsequent diametric tolerances of the tools, and of the nut plate features, is very high because of the number of parameters affecting applied expansion. There are five elements in the applied expansion: inside and outside diameters of the bushing element, diameter of the mandrel, thickness of the sleeve, diameter of the starting hole and the associated tolerances for each.
Most nut plate designs are made to accommodate a range of fastener “fits”. The fit is the amount of clearance between the bolt diameter and the inside diameter of the bushing element of the nut plate. These fits range from a fairly tight “net” fit (0.000 to 0.002 inches) to a very loose fit (up to 0.030 inches). In general, a tight fastener fit is associated with structure requiring load transfer carrying capability of the panels, whereas a loose fit is associated with structure requiring a high degree of interchangeability with panels from another aircraft, vendor, manufacturing lot, or the like.
The novel, improved nut plate installation method disclosed herein uses no split sleeves and therefore has no shear discontinuity to contend with. As a result the applied expansion can be increased to levels much higher than are utilized in a split sleeve nut plate design. Even with relaxed tolerances, the novel, improved nut plate designs disclosed herein exhibits greater resistance to push-out and rotation.
Also, the improved nut plate design disclosed herein does not require as high a material strength as the split sleeve installed nut plate, in order to function properly. This is especially important because the improved nut plate design allows for the use of aluminum nut plates. Such nut plates are significantly lighter than their prior art steel and titanium counterparts and have an added bonus in that prior art materials using dissimilar metals often incurred corrosion problems in aluminum walls, which difficulties can be avoided by use of similar materials.
In view of the close tolerance requirements involved in prior art nut plate designs, and the fact that prior art fatigue life enhancing methods involve the use of complicated processes such as cold expansion or split sleeve methods, it can readily be appreciated that a simple and novel method for installing nutplates, and the unique fatigue life enhanced structures including such nutplates as disclosed hereinbelow, represent important improvements in the art.
The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual implementations depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of unique method and apparatus for securing a blind nut attachment are also shown and briefly described to enable the reader to understand how various features, including optional or alternate features, may be utilized in order to provide an efficient, fatigue resistant, low cost nut plate design, which can be manufactured in a desired size and configuration for providing a long lasting component.